This application claims the entire benefit of Japanese Patent Application Number 2008-078613 filed on Mar. 25, 2008, the entirety of which is incorporated by reference.
1. Field of the invention
The present invention relates to an impact tool, such as an impact driver, which causes an anvil protruding from a front side of a housing to generate a rotary impact force.
2. Description of related art
As disclosed in Japanese Laid-open Patent Publication No. 2003-231067, an impact tool such as an impact driver includes: a spindle disposed in a housing and configured to be rotated by a motor; an anvil disposed in front of the spindle and rotatably supported in the housing in a manner coaxial with the spindle, a front end of the anvil having an insertion hole for attaching a bit and protruding from a front side of the housing; and an impact mechanism configured to transmit a rotation of the spindle as a rotary impact force to the anvil, and an action of the impact mechanism caused by a rotation of the spindle is transmitted to the anvil as a rotary impact force.
In this impact mechanism, the rear end of the spindle has a large-diameter portion for supporting planetary gears which are meshed with an output shaft of the motor. A hammer is fitted onto the spindle at a front side portion of the spindle via balls, which are received between longitudinal grooves formed on the inner surface of the hammer and cam grooves formed on the outer surface of the spindle. This configuration allows the hammer to be rotatable along with the spindle as well as to be movable in the front-and-rear direction with a predetermined stroke length. Further, a coil spring is positioned between the large-diameter portion at a rear side of the spindle and the hammer, so as to urge the hammer toward the advanced position at which the hammer is engaged with the anvil. It is noted that the front surface of the large-diameter portion of the spindle has a circular engagement projection which protrudes from a center of the front surface to receive the rear end of the coil spring. A washer is inserted onto the circular engagement projection so as to restrict the inner diameter of the coil spring at the rear end of the coil spring. Meanwhile, a ring-shaped groove is formed at the rear surface of the hammer to receive the front end of the coil spring. The front end of the coil spring is loosely fitted into the ring-shaped groove, so as to restrict the inner diameter of the coil spring at its front end.
When the motor is driven and the spindle rotates, a rotation of the spindle is transmitted to the hammer via the balls and causes the hammer to rotate, so that the anvil engaged with the hammer also rotates. Therefore, a screw-tightening operation, etc. can be performed using a bit attached to the front end of the anvil. As the screw-tightening operation proceeds and a load applied to the anvil increases to a certain threshold, the hammer retreats or moves backward along the cam grooves against the urging force of the coil spring. Once the hammer retreats and disengages from flanges of the anvil, the hammer is again advanced forward by the urging force of the coil spring while rotating along with the spindle. The hammer is then reengaged with the flanges of the anvil. Accordingly, the hammer moves back and forth repeatedly to repeat disengagement from and reengagement with the flanges of the anvil, so that an intermittent rotary impact force to the anvil is provided for a retightening function of the impact driver.
In this type of the impact tool, in order to improve usability and cost, there has been an increased need to reduce the size of the impact tool, especially the length along the axial direction of the anvil. However, as long as the impact tool includes the aforementioned impact mechanism, it is difficult to reduce the size of the impact tool. This is because the impact tool requires a space in the housing to house the impact mechanism including a space for a stroke movement of the hammer, and also because design variation for changing the mass of the hammer, the shape of the anvil, etc. will be restricted in order to ensure sufficient impacting capability.
In view of the above drawback of the prior art, the present invention seeks to provide an impact tool, which is simple in structure and can reduce the size of the impact tool along the axial direction of the anvil.
The present invention has been made in an attempt to eliminate the above disadvantages, and illustrative, non-limiting embodiments of the present invention overcome the above disadvantage and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantage described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.